Non-aqueous electrolyte secondary batteries can realize high voltage and high energy density, so they have been researched extensively. Positive electrodes of non-aqueous electrolyte secondary batteries include a transition metal oxide or a transition metal chalcogenide, such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2, or MoS2. Such material has a layered or tunnel-like crystal structure into and from which lithium ions are intercalated and deintercalated. The negative electrodes typically include a carbon material. Although a carbon material typically has a relatively small capacity, it is capable of reversibly absorbing and desorbing lithium, thereby providing a battery that is excellent in terms of cycle life and safety. Thus, lithium ion batteries including a negative electrode using a graphite type carbon material have been commercialized.
However, the theoretical capacity of a graphite material is 372 mAh/g, and the theoretical density thereof is 2.2 g/cm3, both of which are relatively small. It is therefore desired that a metal material capable of realizing a higher capacity than that of a graphite material will be used for a negative electrode. Among metal materials, Si particularly has a high capacity of 4199 mAh/g (theoretical density: 2.33 g/cm3), and hence an extensive research and development studies have been underway.
Although Si is capable of realizing a high capacity negative electrode, it has a significant problem to be solved with respect to the charge/discharge cycle characteristics of the resultant battery. The problem is that during charge and discharge reactions, the absorption and desorption of lithium involves repeated expansion and contraction of Si, thereby increasing the contact resistance among particles in the negative electrode and degrading the current collecting network. Such degraded current collecting network becomes a major factor that causes shortening of charge/discharge cycle life.
In order to solve the above-mentioned problems, there has been proposed as a negative electrode material an alloy forming material (hereinafter referred to as “alloying material”) that is capable of reversibly absorbing and desorbing lithium and includes a solid phase A and a solid phase B. The solid phase A and the solid phase B have a different composition, and at least part of the solid phase A is coated with the solid phase B. The solid phase A contains silicon, tin, and/or zinc, while the solid phase B contains a Group 2A element, a transition element, a Group 2B element, a Group 3B element, and/or a Group 4B element. Also, according to this proposal, the solid phase A is preferably amorphous or of low crystallinity. See, for example, U.S. Pat. No. 6,090,505 and Japanese Laid-Open Patent Publication No. 2004-103340.
The related art techniques mentioned above are capable of significantly suppressing the cracking of the alloying material upon expansion and contraction thereof. Therefore, these techniques are effective to a certain extent in suppressing the degradation of the current collecting network, which is a major factor causing the degradation of cycle characteristics. However, a detailed examination of these related art techniques has revealed that the techniques may not produce sufficient suppression of cycle characteristic deterioration when batteries are rapidly charged and discharged at a relatively large current.